1. Field of the Invention
The present invention relates to a mobile communication system for executing a hard handover, which performs communication using different frequencies before and after the handover, under an environment which is supporting a mixed usage of a plurality of frequency bands or frequencies. In particular, this relates to a mobile communication system which can suppress the execution of a compressed mode as much as possible when selecting a neighbor cell which becomes a candidate of the handover partner, and can control the selection of a frequency to be used after the handover by a mobile communication system itself.
2. Description of the Related Art
An IMT (International Mobile Communication)-2000 mobile communication system can be developed in an environment which is supporting a mixed usage of a plurality of frequency bands or a plurality of frequencies. Then, in 3GPP (3rd Generation Partnership Project), available frequency bands in a mobile communication system are specified as shown in FIG. 24 as UTRAN (UMTS Terrestrial Radio Access Network) FDD (Frequency Division Duplex) Frequency bands.
With referring to FIG. 25, six frequency bands (Operating Bands) of 1 to 6 are available. For each frequency band, UL (uplink) Frequencies and DL (downlink) Frequencies are assigned to an uplink and a downlink, respectively. The uplink is a link through which mobile equipment (UE: user equipment) performs signal transmission and a base station (Node-B) performs signal reception, and the downlink is a link through which the Node-B performs signal transmission and the UE performs signal reception.
It is determined by every country or region how to assign the actual frequency bands defined in the 3GPP to communication common carriers (Communication Administrators or Network Operators: hereafter, these are called network operators) or systems. In addition, it is permitted to use these frequency bands are mixed in the same area.
Furthermore, a UARFCN (UTRA Absolute Radio Frequency Channel Number) corresponding to a carrier frequency used within each frequency band is defined by the 3GPP.
FIG. 26 is a table showing relation between the UARFCN and carrier frequencies for general channels which are specified by the 3GPP. FIG. 27 is a table showing relation between the UARFCN and carrier frequencies for additional channels which are specified by the 3GPP. In the IMT-2000 mobile communication system, the UARFCN defined in FIGS. 26 and 27 are used in respective frequency bands 1 to 6 as shown in FIG. 28.
In addition, the UE of the IMT-2000 mobile communication system measures signal level of the CPICH (Common Pilot Channel) of frequency bands used by neighbor cells which become candidates of the handover partner, so as to determine whether the hard handover can be performed. In this case, the neighbor cells may be using a different frequency band from the frequency band currently used by the UE in the environment which permits a mixed usage of a plurality of frequency bands and frequencies.
At this time, a radio network controller (RNC) makes a Node-B and the UE activate a compressed mode, if needed.
The compressed mode is to make a transmission gap (idle time) by compressing a transmission interval of transmission data at the time of transmitting the transmission data. Then, the UE changes a received frequency band during this transmission gap, and measures signal level of the CPICH of other frequency bands. What is well known as a method of the compressed mode is to increase transmission speed temporarily by making an SF (Spreading Factor: diffusivity) one half during compression transmission so that it may be possible to transmit the same number of bits, as those to be transmitted in an ordinary time, in a slot other than the transmission gap. In this method, transmission power is strengthened temporarily during compressed transmission since a transmission bit rate has been increased. When transmission power becomes large, signal interference between channels increases, and it means that an available channel number decreases consequently. As the result, the radio capacity drops.
In addition, as operations in the compressed mode, three kinds of compressed mode operations are defined. They are, a method of making both of the uplink and the downlink set in the compressed mode, a method of making only the uplink set in the compressed mode, and a method of making only the downlink set in the compressed mode.
When the UE has only one receiver, i.e., a single receiver, it is necessary to be made set in the compressed mode in order to measure a different frequency. The different frequency means a frequency different from a frequency currently being used by the UE. When the compressed mode in the uplink is unnecessary, the UE stops downlink signal reception with continuing uplink signal transmission, and measures a different frequency. When both links of the uplink and the downlink need to be made set into the compressed mode, the UE stops usual transmission and reception, and measures a different frequency.
When the UE has two receivers, i.e., a dual receiver, the compressed mode may be not necessary. On the other hand, also in the case of the dual receiver, depending on a frequency which is measured, the compressed mode in the uplink may be needed. When making only the uplink set in the compressed mode, the UE stops usual transmission and measures a different frequency with one receiver.
For example, the UE with dual receiver can receive two frequencies simultaneously while transmitting data. Here, it is assumed that the UE is a dual receiver terminal which supports frequency bands 1, 3, and 5. As shown in FIG. 25, the frequency band 1 is 1920 to 1980 MHz in the uplink, and is 2110 to 2170 MHz in the downlink. The frequency band 3 is 1710 to 1785 MHz in the uplink and is 1805 to 1880 MHz in the downlink. The frequency band 5 is 824 to 849 MHz in the uplink and is 869 to 894 MHz in the downlink.
In this case, the dual receiver terminal can receive simultaneously the frequency band 1 and the frequency band 5, whose operating frequencies are separated each other, without using the compressed mode. Therefore, the UE, which is locating in an area of the frequency band 1, can measure the frequency band 5 even if not using the compressed mode.
However, when the UE, which is locating in the area of the frequency band 1, measures the frequency band 3, frequencies are close in the case of the downlink (1805 to 1880 MHz) of the frequency band 3, and the uplink (1920 to 1980 MHz) of the frequency band 1. Hence, there is a possibility that transmission radio signal of the UE itself affects the measurement of the downlink radio signal if transmission of the UE is not stopped. Therefore, it is necessary to perform the compressed mode for the uplink. Similarly, when the UE locating in the area of the frequency band 3 measures the frequency band 3, and also when the UE locating in the area of the frequency band 5 measures the frequency band 5, it is supposed that the compressed mode only in the uplink is needed.
These three kinds of compressed modes are properly used according to the construction and operation of the UE, such as availability of the dual receiver, and interlock operation of the transmitter and the receiver.
Furthermore, according to the 3GPP specifications, it is defined as a restriction of different frequency measurement that the UE is to measure up to two different frequencies. Therefore, even if the RNC instructs the UE to measure three or more frequencies in a cell, the UE measures up to two frequencies, and does not measure the remaining frequencies.
In addition, the necessity of the compressed mode activation at the time of the different frequency measurement in the UE has been determined beforehand as a UE capability indication for every combination of an operating frequency band and a measurable frequency band by the UE. When the UE establishes an RRC (Radio Resource Control) connection to the RNC before communication start, the content of the UE capability indication is reported to the RNC by the RRC CONNECTION SETUP COMPLETE message of the RRC protocol.
The RNC designates the Node-B and the UE to activate the compressed mode on the basis of the UE capability indication, which has been reported beforehand from the UE, if necessary, when the UE performs the different frequency measurement.
In addition, Japanese Patent Laid-Open No. 2003-078936 discloses the handover system technology which performs the handover to a cell using a different frequency by the single receiver UE with suppressing the execution of the compressed mode as much as possible without measuring a pilot signal other than a frequency signal for communication. According to this technology, the handover is executed as follows. First, when the UE under communication with another frequency approaches a cell using a different frequency, a radio signal at the frequency for communication which is transmitted by the UE is detected as jamming in the Node-B of the cell. Therefore, the Node-B of the cell requests the activation of the compressed mode to the RNC. When the designation of the compressed mode is issued, the Node-B executes the compressed mode in the downlink to make it possible for the UE to receive a frequency of the Node-B. As a result, the UE knows own approach to the cell at the different frequency, and performs the handover. In addition, by performing control so as to execute the compressed mode intermittently, the execution of the compressed mode is suppressed as much as possible.
As explained above, it is specified that, even if the IMT-2000 mobile communication system is developed in the environment which permits the mixed usage of a plurality of frequency bands or a plurality of frequencies, it is possible to measure the different frequency by executing the compressed mode and to perform the handover. However, it was not possible that the mobile communication system itself can flexibly control the selection of the frequency band or frequency for the UE to perform the handover.
Therefore, it was necessary to assure an inter-frequency hard handover to all the frequency bands which the UE supported. Hence, the RNC confirmed all the frequency bands included in the UE capability indication reported from the UE to activate the compressed mode when the compressed mode was required for at least one frequency band which the UE supported. As a result, even a case where the compressed mode might have been unnecessary in an certain combination of measurement frequencies, the compressed mode was activated, and in consequence, there was a problem that signal interference between channels was likely to increase and radio capacity decreased.